Highly Stereoregular Syndiotactic Polypropylene Formation with Metallocene Catalysts via Influence of Distal Ligand Substituents

Highly stereoregular syndiotactic polypropylene is obtained with the catalyst systems Ph_2C(Oct)(C_5H_4)ZrCl_2/MAO (8/MAO) (Oct = octamethyloctahydrodibenzofluorenyl; MAO = methylaluminoxane) and Me_2C(Oct)(C_5H_4)ZrCl_2/MAO (12/MAO). The syndiotactic polypropylenes obtained are largely devoid of stereoerrors by ^(13)C NMR analysis ([r] > 98%), and melting temperatures as high as 153 or 154 °C (from 8 and 12, respectively) are found for the thermally quenched polymers (without annealing). A related hafnium catalytic system, Ph_2C(Tet)(C_5H_4)HfCl_2/MAO (11/MAO) (Tet = tetramethyltetrahydrobenzofluorenyl), was found to be the most syndioselective of the hafnocenes tested (T_m = 141 °C). The metallocene dichloride precatalysts represent the first examples of transition metal complexes containing the Oct or Tet ligands. Reported are the solid state crystal structures of 8, the diprotio ligand precursor of 8(Ph_2C(OctH)(C_5H_5)), and the zirconium analogue of 11, Ph_2C(Tet)(C_5H_4)ZrCl_2 (10). Distal ligand influences are thus demonstrated to have a dramatic effect on polymer stereochemistry